Polytetrafluoroethylene (PTFE) homopolymers and related modified PTFE polymers have exceptional stability to light, heat, solvents, chemical attack and electrical stresses, conferring desirable properties to articles made from these polymers. But because of the inability to melt process these polymers and the difficulties associated with solution processing, it is very difficult to spin or shape them by conventional methods. Therefore other processes have been developed for preparing fibers of PTFE homopolymers and modified PTFE.
Dispersion spinning is one method developed for producing shaped articles such as fibers from fluorinated polymers. Non-melt processible fluoropolymers may be successfully spun from a mixture of an aqueous dispersion of fluorinated polymer particles mixed with a solution of a suitable matrix polymer. An intermediate structure is formed when this mixture is contacted with a suitable coagulation bath. Although the intermediate structure is mechanically sound, a final, sintered structure is generally formed by heating the intermediate structure to a temperature sufficient to coalesce the fluorinated polymer particles. On sintering the matrix polymer decomposes to form volatile gases and a carbonaceous residue.
Fluorosurfactants are typically used in the dispersion polymerization of fluoropolymers, the fluorosurfactants functioning as a non-telogenic dispersing agent, as described in U.S. Pat. No. 2,559,752 to Berry. Unless removed, fluorosurfactant is present in fluoropolymer dispersions and is present in the fiber spinning compositions made from such dispersions. However because of environmental concerns, processes have been developed to reduce the fluorosurfactant content in aqueous fluoropolymer dispersions to decrease emissions of fluorosurfactants and/or decrease or eliminate the need to capture fluorosurfactants during end use processing of fluoropolymer dispersions.
Several techniques are known for the reduction of fluorosurfactant content from fluoropolymer dispersions such as concentration by ultrafiltration as described in U.S. Pat. No. 4,369,266 (Kuhls et al.) and contact with ion exchange resin as described in U.S. Pat. No. 3,882,153 (Seki et al), U.S. Pat. No. 4,282,162 (Kuhls) and US 2003/0125421 A1 (Bladel et al.).
Fluoropolymer dispersions are typically subjected to a concentration step after manufacture to increase their concentration. Nonionic surfactants are usually added prior to concentration to increase the stability of the dispersion. For use in known fiber spinning processes, fluoropolymer dispersions containing alkyl phenol ethoxylates have been used for stabilizing and concentrating fluoropolymer dispersions. However, alkyl phenol ethoxylates are prone to causing smoking during sintering of the fiber, causing foaming in wash water, and forming deposits on rolls which contact the fiber. In addition, because of environmental concerns about compounds containing aromatic groups, it is desirable to avoid alkyl phenol ethoxylates which contain aromatic groups.
An improved fluoropolymer spinning process and spinning composition are desired that employ fluoropolymer dispersions with reduced fluorosurfactant content and which overcome problems associated with alkyl phenol ethoxylate nonionic surfactants.